The present invention relates to biochemical transformation of solid carbonaceous material by a process of depolymerizing, desulfurizing and demineralizing using modified strains of thermophilic bacteria. More specifically, the present invention provides a process of treating an aqueous slurry of solid carbonaceous material, such as coal, with a culture of thermophilic bacteria which have been modified through challenge growth processes to be suitable for microbially enhanced oil recovery. Some of these bacteria were further adapted to grow on coal under challenged growth conditions and then utilized for biochemical transformation of solid carbonaceous material.
Coal is a solid carbonaceous material which is one of the most abundant fossil energy resources accessible to mankind. In terms of environmental protection, coal combustion is associated with significant problems, such as sulfur and nitrogen oxides emission, and the production of toxic metal containing ash. However, coal can be treated prior to combustion in a manner which reduces sulfur and ash contents. The treated product becomes a more suitable feedstock for combustion, liquefaction and gasification processes.
Sulfur appears in coal in three basic forms: as sulfates, pyrites and organic sulfur. Of the three forms, sulfates are the least significant, comprising less than 0.5 weight percent of coal. Pyritic and organic sulfur, however, may each constitute as much as 3.5 weight percent of the coal or from 40% weight to 60% weight of the total sulfur content, respectively. Thus, it is apparent that removal of an effective portion of inorganic and organic sulfur content prior to coal combustion could substantially reduce the emission of sulfur oxides into the atmosphere.
By organic sulfur is meant sulfur which is chemically bound within the coal matrix. Organic sulfur is present in four major forms. These are mercaptans or thiols, sulfides, disulfides and aromatic ring sulfur as exemplified by the thiophene system.
In the past, attempts have been made to alleviate the environmental problems associated with the combustion of coal. A number of processes for the treatment of coal ranging from ash and pyrite reduction to organic sulfur removal and coal liquefaction have been reported. These processes include numerous physical and mechanical techniques such as heavy media separation, elective agglomeration, flotation, jigging, magnetic, separation, leaching and hydrosulfurization. Problems associated with these developing technologies include reproductivity, slow kinetics and scale-up difficulties.
The removal of inorganic sulfur from pyrite is relatively easily accomplished by treatment with dilute nitric acid. The removal of organic sulfur, however, has not met with the same success. In recent years in order to obtain cleaner coal, coal has been subjected to biochemical processes wherein inorganic sulfur and minerals are removed through the action of several microorganisms belonging to the Thiobacillus, Sulfolobus and Pseudomonas Clostridium species. For example, U.S. Pat. No. 4,632,906 to Kopacz discloses a process for biodesulfurization by contacting carbonaceous material with Bacillus Sulfasportate of ATCC No. 39909. A mutant of the family Bacillaceae, this microorganism grows in a range between 15.degree. C. to 30.degree. C. and is not a thermophile. The background of the invention of the '906 disclosure describes Sulfolobus Acidocaldarius, a thermophilic sulfur and iron oxidizing microorganism which is a facultative autotroph. This organism has been used for removal of primarily pyritic sulfur from coal.
U.S. Pat. No. 4,206,288 to Detz et al. discloses a process for removal of pyritic sulfur from coal by microbial desulfurization of coal slurry with iron and sulfur oxidizing microorganisms selected from the Thiobacillus ferooxidans species. The microorganisms described in the '288 disclosure must be kept in a range of about 10.degree. C. to 35.degree. C. and are not thermophilic as required by the process of the present invention. Thus, they cannot be used at elevated temperatures.
U.S. Pat. No. 4,562,156 to Isbister et al. describes a mutant microorganism Pseudomonas sp. CB1 (ATCC 39381) used in the removal of organic sulfur compounds from carbonaceous materials including coal. In addition, the '156 disclosure describes other genera of microorganisms such as Arthrobacter and Acinetobacter used for microbiological treatment of petroleum and coal. These microorganisms are not thermophiles because they cannot grow at 41.degree. C.
Generally, the microorganisms described in the above disclosures are not resistant to challenged environments, namely those including elevated temperatures, pressures, low pH, high levels of salinity and toxic metals. Frequently, it has been found more economical to process coal under elevated temperatures and pressures. Moreover, many types of coal have high toxic metal content which can kill many strains of bacteria even those which have been previously environmentally challenged.
Accordingly, there is a need in the art of biochemical transformation of solid carbonaceous materials for microorganisms which can depolymerize, desulfurize and/or demineralize solid carbonaceous material such as coal under environmentally challenged conditions.
It is therefore, an object of the present invention to provide modified thermophilic bacteria which are useful for the biochemical transformation of coals in a challenged environment.